A deranged straphanger stabbed an MTA bus driver in the arm with a syringe today in Brooklyn, authorities said.

The bus was on Prospect Park West near Greenwood Avenue at 11 a.m. when the sicko lashed out, said a spokesman for the FDNY.

The man then fled from the bus, officials said.

The bus driver was taken to Methodist Hospital, said a spokesman for the FDNY.